Endoscope objective optical sysem

ABSTRACT

This endoscope objective optical system is capable of acquiring images of high resolution and wide angle of observation field, maintaining low invasiveness and appropriately correcting various aberrations. This optical system has at least a first cemented lens which has a positive lens and a negative lens, in which the cemented lens satisfies the following conditional expressions: (1) 15.0&lt;νA−ndA&lt;15.75 and (2) −0.2&gt;rdyA1/ih&gt;−20, wherein νA is an Abbe number of the negative lens, ndA is a refractive index of the negative lens at the d-line, rdyA1 is a curvature radius of a joining surface of the negative lens, and ih is an image height.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. patent applicationSer. No. 15/019,264, filed on Feb. 9, 2016, which is a ContinuationApplication of International Application No. PCT/JP2014/071652, filed onAug. 19, 2014, which claims priority to Japanese Application No.2013-172239, filed on Aug. 22, 2013, the contents of which are herebyincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an objective optical system and, moreparticularly, to an endoscope objective optical system to be applied toa medical endoscope.

BACKGROUND ART

As a medical endoscope, it is preferable to acquire an image of a highresolution and a wide angle of observation field, while ensuring lowinvasive to the patient. However, high image quality and less invasiveare opposite to each other. Specifically, in order to obtain a highquality image, it is necessary to increase the number of pixels, and itis preferable to use a large imaging element. On the other hand, as thesize of the imaging element increases, the imaging lens diameter becomeslarge, and as a result, the external diameter becomes larger, it makesit difficult to ensure the less invasiveness.

Therefore, in recent years, the pixel pitch is made smaller, the numberof pixels is increased without increasing the size of the imagingelement, a method for obtaining high quality images are becoming themainstream. The pixel pitch has been becoming smaller and smaller astime goes and imaging elements having a pixel pitch of a few microns orless have also been developed.

For example, Patent Literature 1 and Patent Literature 2 disclose anobjective optical system adapted to the miniaturized imaging elementwith the above-described pixel pitch which is several microns or less.

CITATION LIST Patent Literature {PTL 1} Japanese Unexamined PatentApplication, Publication No. 2007-249189 {PTL 2} Japanese UnexaminedPatent Application, Publication No. 2011-247949 SUMMARY OF INVENTION

One aspect of the present invention is an endoscope objective opticalsystem including at least a first cemented lens which has a positivelens and a negative lens, in which the cemented lens satisfies thefollowing conditional expressions,

15.0<νA−ndA<15.75  (1)

−0.2>rdyA1/ih>−20  (2)

wherein νA is an Abbe number of the negative lens, ndA is a refractiveindex of the negative lens at the d-line, rdyA1 is a curvature radius ofa joining surface of the negative lens, and ih is an image height.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of the overall structure of anobjective optical system according to a first embodiment of the presentinvention.

FIG. 2 is a cross-sectional view of the overall structure of anobjective optical system according to a second embodiment of the presentinvention.

FIG. 3 is a cross-sectional view of the overall structure of anobjective optical system according to a third embodiment of the presentinvention.

FIG. 4 is a cross-sectional view of the overall structure of anobjective optical system according to Example 1 of the presentinvention.

FIG. 5 shows graphs of aberrations of the objective optical systemaccording to Example 1 of the present invention.

FIG. 6A depicts cross-sectional views of the overall structure of anobjective optical system according to Example 2 of the present inventionwhen the optical system is in a normal view state.

FIG. 6B depicts cross-sectional views of the overall structure of anobjective optical system according to Example 2 of the present inventionwhen the optical system is in a magnified view state.

FIG. 7 shows graphs of aberrations in the normal view state in theobjective optical system according to Example 2 of the presentinvention.

FIG. 8 shows graphs of aberrations in the magnified view state in theobjective optical system according to Example 2 of the presentinvention.

FIG. 9A depicts cross-sectional views of the overall structure of anobjective optical system according to Example 3 of the present inventionwhen the optical system in a normal view state.

FIG. 9B depicts cross-sectional views of the overall structure of anobjective optical system according to Example 3 of the present inventionwhen the optical system is in a magnified view state.

FIG. 10 shows graphs of aberrations in the normal view state in theobjective optical system according to Example 3 of the presentinvention.

FIG. 11 shows graphs of aberrations in the magnified view state in theobjective optical system according to Example 3 of the presentinvention.

FIG. 12A depicts cross-sectional views of the overall structure of anobjective optical system according to Example 4 of the present inventionwhen the optical system is in a normal view state.

FIG. 12B depicts cross-sectional views of the overall structure of anobjective optical system according to Example 4 of the present inventionwhen the optical system is in a magnified view state.

FIG. 13 shows graphs of aberrations in the normal view state in theobjective optical system according to Example 4 of the presentinvention.

FIG. 14 shows graphs of aberrations in the magnified view state in theobjective optical system according to Example 4 of the presentinvention.

FIG. 15 is a cross-sectional view of the overall structure of anobjective optical system according to Example 5 of the presentinvention.

FIG. 16 shows graphs of aberrations of the objective optical systemaccording to Example 5 of the present invention.

FIG. 17 is a cross-sectional view of the overall structure of anobjective optical system according to Example 6 of the presentinvention.

FIG. 18 shows graphs of aberrations of the objective optical systemaccording to Example 6 of the present invention.

FIG. 19A depicts cross-sectional views of the overall structure of anobjective optical system according to Example 7 of the present inventionwhen the optical system is in a normal view state.

FIG. 19B depicts cross-sectional views of the overall structure of anobjective optical system according to Example 7 of the present inventionwhen the optical system is in a magnified view state.

FIG. 20 shows graphs of aberrations in the normal view state in theobjective optical system according to Example 7 of the presentinvention.

FIG. 21 shows graphs of aberrations in the magnified view state in theobjective optical system according to Example 7 of the presentinvention.

FIG. 22 is a cross-sectional view of the overall structure of anobjective optical system according to Example 8 of the presentinvention.

FIG. 23 shows graphs of aberrations of the objective optical systemaccording to Example 8 of the present invention.

FIG. 24 is a cross-sectional view of the overall structure of anobjective optical system according to Example 9 of the presentinvention.

FIG. 25 shows graphs of aberrations of the objective optical systemaccording to Example 9 of the present invention.

FIG. 26 is a cross-sectional view of the overall structure of anobjective optical system according to Example 10 of the presentinvention.

FIG. 27 shows graphs of aberrations of the objective optical systemaccording to Example 10 of the present invention.

FIG. 28 is a cross-sectional view of the overall structure of anobjective optical system according to Example 11 of the presentinvention.

FIG. 29 shows graphs of aberrations of the objective optical systemaccording to Example 11 of the present invention.

FIG. 30 is a cross-sectional view of the overall structure of anobjective optical system according to Example 12 of the presentinvention.

FIG. 31 shows graphs of aberrations of the objective optical systemaccording to Example 12 of the present invention.

FIG. 32A depicts cross-sectional views of the overall structure of anobjective optical system according to Example 13 of the presentinvention when the optical system is in a normal view state.

FIG. 32B depicts cross-sectional views of the overall structure of anobjective optical system according to Example 13 of the presentinvention when the optical system is in a magnified view state.

FIG. 33 shows graphs of aberrations in the normal view state in theobjective optical system according to Example 13 of the presentinvention.

FIG. 34 shows graphs of aberrations in the magnified view state in theobjective optical system according to Example 13 of the presentinvention.

FIG. 35A depicts cross-sectional views of the overall structure of anobjective optical system according to Example 14 of the presentinvention when the optical system is in a normal view state.

FIG. 35B depicts cross-sectional views of the overall structure of anobjective optical system according to Example 14 of the presentinvention when the optical system is in a magnified view state.

FIG. 36 shows graphs of aberrations in the normal view state in theobjective optical system according to Example 14 of the presentinvention.

FIG. 37 shows graphs of aberrations in the magnified view state in theobjective optical system according to Example 14 of the presentinvention.

FIG. 38A depicts cross-sectional views of the overall structure of anobjective optical system according to Example 15 of the presentinvention when the optical system is in a normal view state.

FIG. 38B depicts cross-sectional views of the overall structure of anobjective optical system according to Example 15 of the presentinvention when the optical system is in a magnified view state.

FIG. 39 shows graphs of aberrations in the normal view state of theobjective optical system according to Example 15 of the presentinvention.

FIG. 40 shows graphs of aberrations in the magnified view state in theobjective optical system according to Example 15 of the presentinvention.

DESCRIPTION OF EMBODIMENTS First Embodiment

An objective optical system according to a first embodiment of thepresent invention will be described below with reference to thedrawings.

FIG. 1 is a cross-sectional view of the overall structure of theobjective optical system. As shown in FIG. 1, the objective opticalsystem includes a front lens group GF, an aperture stop S, and a rearlens group GR in this order from an object.

The front lens group GF includes a negative first lens L1 and a positivesecond lens L2 in this order from the object-side plane, and haspositive refracting power. The rear lens group GR includes a parallelflat plate F and a cemented lens CL1 (first cemented lens) formed byjoining a positive third lens L3 and a negative fourth lens L4, and haspositive refracting power.

The cemented lens CL1 is formed so as to satisfy the followingconditional expression (1) and conditional expression (2).

15.0<νA−ndA<15.75  (1)

−0.2>rdyA1/ih>−20  (2)

In the expression, νA is an Abbe number of the negative lens of thecemented lens CL1, ndA is a refractive index of the negative lens at thed-line, rdyA1 is a curvature radius of the joining surface of thenegative lens of the cemented lens CL1, and ih is an image height.

When the upper limit of the conditional expression (1) is exceeded, therefractive index of the negative lens is too small to achieve necessarynegative refracting power. In order to achieve negative refractingpower, the curvatures of the joining surface and the air-contactingsurface need to be increased, which undesirably causes, in particular,non-axial aberration. When the lower limit of the conditional expression(1) is exceeded, the Abbe number of the negative lens is too small,which leads to a state in which axial and non-axial chromatic aberrationcan be easily caused.

Furthermore, when the upper limit of the conditional expression (2) isexceeded, the curvature of the cemented lens becomes too small and hencethe color correction effect of the cemented lens becomes lessened, whichleads to a state in which axial and non-axial chromatic aberration canbe easily caused. When the lower limit of the conditional expression (2)is exceeded, the curvature of the cemented lens becomes too large, whichleads to a state in which axial and non-axial chromatic aberration canbe easily caused.

Therefore, it is more preferable that the conditional expression (1)′and the conditional expression (2)′ below or the conditional expression(1)″ and the conditional expression (2)″ below are adopted, assubstitute for the conditional expression (1) and the conditionalexpression (2).

15.3<νA−ndA<15.7  (1)′

−1.0>rdyA1/ih>−5.0  (2)′

15.5<νA−ndA<15.6  (1)″

−1.2>rdyA1/ih>−2.5  (2)″

The negative fourth lens L4 of the cemented lens CL1 is formed so as tosatisfy the following conditional expression (3).

−0.2>(rdyA1+rdyA2)/(rdyA1−rdyA2)>−10  (3)

In the expression, rdyA2 is the curvature radius of the air-contactingsurface of the negative lens of the cemented lens CL1.

The conditional expression (3) is a conditional expression regarding theshape factor of the negative fourth lens L4 of the cemented lens CL1. Asa result of the negative fourth lens L4 of the cemented lens CL1satisfying the conditional expression (3), axial and non-axial chromaticaberration can be corrected while still achieving necessary negativerefracting power. When the upper limit of the conditional expression (3)is exceeded, the curvature radius of the joining surface becomes toosmall, which may make the processing difficult. In addition, because thecurvature of the positive third lens L3 of the cemented lens CL1 alsobecomes large, it becomes difficult to secure a sufficient thickness ofthe positive third lens L3 at the periphery thereof. When the lowerlimit of the conditional expression (3) is exceeded, the curvatureradius of the joining surface becomes too large, which makes itdifficult to correct axial and non-axial chromatic aberration.

For this reasons above, it is more preferable that the conditionalexpression (3)′ or the conditional expression (3)″ below are adopted, assubstitute for the conditional expression (3).

−0.3>(rdyA1+rdyA2)/(rdyA1−rdyA2)>−3.0  (3)′

The following conditional expression is still more preferable.

−0.4>(rdyA1+rdyA2)/(rdyA1−rdyA2)>−2.5  (3)″

It is preferable that in the objective optical system according to thisembodiment, the negative lens is disposed most closely to theobject-side plane, like the first lens L1 of the front lens group GF inFIG. 1, and the negative lens disposed most closely to the object-sideplane is configured so as to satisfy the following conditionalexpression (4).

−3.0≤rdy12/rdyA1<−0.2  (4)

In the expression, rdy12 is the image-side curvature radius of thenegative first lens.

The conditional expression (4) is a conditional expression for theimage-side curvature radius of the negative first lens L1 and thecurvature radius of the cemented lens CL1. By satisfying the conditionalexpression (4), it is possible to favorably maintain the balance betweenthe image-side curvature radius of the negative first lens L1 and thecurvature radius of the cemented lens CL1, making it possible tofavorably correct comatic aberration, field curvature, axial chromaticaberration, and chromatic aberration of magnification. However, when theupper limit of the conditional expression (4) is exceeded, the curvatureradius of the negative first lens L1 becomes large, which may worsencomatic aberration, field curvature, and distortion. Furthermore, whenthe lower limit of the conditional expression (4) is exceeded, thecurvature radius of the cemented lens CL1 becomes too large, which makesit difficult to correct axial chromatic aberration and chromaticaberration of magnification.

Therefore, it is more preferable that the conditional expression (4)′ orthe conditional expression (4)″ below are adopted, as substitute for theconditional expression (4).

−2.5≤rdy12/rdyA1<−0.3  (4)′

−2.0≤rdy12/rdyA1<−0.39  (4)″

Furthermore, the cemented lens CL1 is configured to satisfy thefollowing conditional expression (5).

1.0<DB/DA<10  (5)

0.1<DA/ih<2.0  (6)

In the expression, DA is the thickness at the middle of the negativefourth lens L4 of the cemented lens CL1, and DB is the thickness at themiddle of the positive third lens L3 of the cemented lens CL1.

The conditional expression (5) and the conditional expression (6) areconditional expressions regarding the thickness at the middle of thecemented lens CL1. By satisfying the conditional expression (5) and theconditional expression (6), it becomes possible to achieve an objectiveoptical system with an appropriate overall length in which the lenses donot easily exhibit manufacturing defects, such as fracture and cracks.

When the upper limit of the conditional expression (5) is exceeded, thethickness at the middle of the negative fourth lens L4 of the cementedlens CL1 becomes too small, which may easily cause fracture or cracks.When the lower limit of the conditional expression (5) is exceeded, thethickness at the middle of the positive third lens L3 of the cementedlens becomes too small, which makes it difficult to secure a sufficientthickness at the periphery thereof and hence drastically degrades theease of processing.

When the upper limit of the conditional expression (6) is exceeded, thethickness at the middle of the negative fourth lens L4 becomes toolarge, causing the overall length to be undesirably large. When thelower limit of the conditional expression (6) is exceeded, the thicknessat the middle of the negative fourth lens L4 becomes too small, whichmay cause fracture or cracks.

Therefore, it is more preferable that the conditional expression (5)′and the conditional expression (6)′ below or the conditional expression(5)″ and the conditional expression (6)″ below are adopted, assubstitute for the conditional expression (5) and the conditionalexpression (6).

2.5<DB/DA<7.5  (5)′

0.15<DA/ih<1.0  (6)′

The following conditional expression is still more preferable.

4.0<DB/DA<4.5  (5)″

0.2<DA/ih<0.7  (6)″

Furthermore, the negative first lens L1 disposed most closely to theobject-side plane and the negative lens of the cemented lens CL1 areconfigured to satisfy the following conditional expression.

0.5<PW1/PW4<10  (7)

In the expression, PW1 is the refracting power of the negative firstlens, and PW4 is the refracting power of the negative lens of thecemented lens.

The conditional expression (7) is a conditional expression for therefracting power of the negative first lens L1 and the refracting powerof the negative fourth lens L4 of the cemented lens CL1. By satisfyingthe conditional expression (7), it is possible to favorably maintain thebalance between the refracting power of the negative first lens L1 andthe refracting power of the negative third lens L3 in the cemented lensCL1, making it possible to favorably correct comatic aberration, fieldcurvature, axial chromatic aberration, and chromatic aberration ofmagnification. When the upper limit of the conditional expression (7) isexceeded, the refracting power of the negative first lens L1 becomes toointense, worsening comatic aberration, field curvature, and distortion.When the lower limit of the conditional expression (7) is exceeded, therefracting power of the negative fourth lens L4 of the cemented lens CL1becomes too intense, making it difficult to correct axial chromaticaberration and chromatic aberration of magnification.

Therefore, it is more preferable that the conditional expression (7)′ orthe conditional expression (7)″ below are adopted, as substitute for theconditional expression (7).

1.5<PW1/PW4<5.0  (7)′

1.58<PW1/PW4<3.0  (7)″

The negative first lens disposed most closely to the object-side planeis configured to satisfy the following conditional expression (8).

0.5<(rdy11+rdy12)/(rdy11−rdy12)<1.7  (8)

In the expression, rdy11 is the object-side curvature radius of thenegative first lens, and rdy12 is the image-side curvature radius of thenegative first lens.

The conditional expression (8) is a conditional expression regarding theshape factor of the negative first lens L1. By satisfying theconditional expression (8), the necessary negative refracting power canbe achieved. When the lower limit of the conditional expression (8) isexceeded, the refracting power of the negative first lens L1 decreases.When the upper limit of the conditional expression (8) is exceeded, thelens productivity drastically decreases.

Therefore, it is more preferable that the conditional expression (8)′ orthe conditional expression (8)″ below is adopted, as substitute for theconditional expression (8).

0.7<(rdy11+rdy12)/(rdy11−rdy12)<1.3  (8)′

The following conditional expression is still more preferable.

0.99<(rdy11+rdy12)/(rdy11−rdy12)<1.01  (8)″

The cemented lens CL1 is configured to satisfy the following conditionalexpression (9).

0.05<(rdyB1+rdyB2)/(rdyB1−rdyB2)<2.0  (9)

In the expression, rdyB1 is the curvature radius of the air-contactingsurface of the positive third lens L3 in the cemented lens CL1, andrdyB2 is the curvature radius of the joining surface of the positivethird lens L3 in the cemented lens CL1.

The conditional expression (9) is a conditional expression regarding theshape factor of the positive third lens L3 of the cemented lens CL1. Bysatisfying the conditional expression (9), an appropriate curvatureradius can be obtained, making it possible to secure a sufficientthickness at the periphery of the lens while still ensuring thenecessary positive refracting power. When the upper limit or the lowerlimit of the conditional expression (9) is exceeded, either one of thecurvature radii becomes too small and a sufficient thickness at theperiphery thereof cannot be ensured, making manufacturing thereofconsiderably difficult.

Therefore, it is more preferable that the conditional expression (9)′ orthe conditional expression (9)″ below is adopted, as substitute for theconditional expression (9).

0.1<(rdyB1+rdyB2)/(rdyB1−rdyB2)<0.5  (9)′

0.13<(rdyB1+rdyB2)/(rdyB1−rdyB2)<0.45  (9)″

As described above, according to this embodiment, it is possible to makethe objective optical system capable of acquiring an image having highprecision and a wide angle of observation field by satisfactorilycorrecting various aberrations while ensuring low invasiveness.

Although the above-described embodiment has been described by way of anexample where the rear lens group GR is configured to include the onecemented lens CL1, the rear lens group GR may be provided with aplurality of cemented lenses. In addition, both of the front lens groupGF and the rear lens group GR may be configured to include the cementedlens.

Second Embodiment

An objective optical system according to a second embodiment of thepresent invention will be described below with reference to thedrawings.

FIG. 2 is a cross-sectional view of the overall structure of theobjective optical system. As shown in FIG. 2, the objective opticalsystem includes a first lens group G1, an aperture stop S, a second lensgroup, and a third lens group G3 in this order from an object.

The first lens group G1 includes the negative first lens L1, theparallel flat plate F, the positive second lens L2, and the firstcemented lens CL1 formed by joining the positive third lens L3 and thenegative fourth lens L4 in this order from the object, and has positiverefracting power.

The second lens group G2 includes a cemented lens CL2 formed by joininga negative fifth lens L5 and a positive sixth lens L6, and has negativerefracting power. Furthermore, the second lens group G2 is movable alongthe optical axis, and it is possible to switch between a normal view anda magnified view by moving the second lens group G2.

The third lens group G3 includes a positive seventh lens L7, a cementedlens CL3 formed by joining a positive eighth lens L8 and a negativeninth lens L9, and the parallel flat plate F, and has positiverefracting power.

The objective optical system according to this embodiment is alsoconfigured to satisfy the conditional expressions (1) through (9) in theabove-described first embodiment. In this case, it is sufficient if atleast one of the cemented lens CL1 and the cemented lens CL2 isconfigured to satisfy each conditional expression.

Furthermore, the objective optical system according to this embodimentis configured to satisfy the following conditional expression (10).

1<FL2G×Δ2G/FL²<200  (10)

In the expression, Δ2G is the absolute value of the displacement of thesecond lens group from a normal view to a close-up magnified view, FL isa focal length of the entire objective optical system in a normal view,and FL2G is a focal length of the second lens group.

The conditional expression (10) is a conditional expression regardingthe displacement of the second lens group G2 from the normal view to theclose-up magnified view. By satisfying the conditional expression (10),an appropriate displacement can be performed and a focus range inaccordance with a technician's intuition can be realized. When the upperlimit of the conditional expression (10) is exceeded, the displacementis too large, causing the overall length to become undesirably large.When the lower limit of the conditional expression (10) is exceeded, thefocus changes with a small displacement, degrading the usability of thesystem by a technician.

Because of this, it is more preferable that the conditional expression(10)′ or the conditional expression (10)″ below are adopted, assubstitute for the conditional expression (10).

3<FL2G×Δ2G/FL²<10  (10)′

4.4<FL2G×Δ2G/FL²<6.0  (10)″

Third Embodiment

An objective optical system according to a third embodiment of thepresent invention will be described below with reference to thedrawings.

FIG. 3 is a cross-sectional view of the overall structure of theobjective optical system. As shown in FIG. 3, the objective opticalsystem includes a first lens group G1, a second lens group, an aperturestop S, and a third lens group G3 in this order from an object.

The first lens group G1 includes the negative first lens L1, theparallel flat plate F, and the positive second lens L2 in this orderfrom the object, and has positive refracting power.

The second lens group G2 includes the positive third lens L3 movable atthe time of focusing so that it is possible to switch between a normalview and a magnified view by moving the second lens group G2.

The third lens group G3 includes the cemented lens CL1 formed by joiningthe positive fourth lens L4 and the negative fifth lens L5, the cementedlens CL2 formed by joining the positive sixth lens L6 and the negativeseventh lens L7, and the parallel flat plate F, and has positiverefracting power.

The objective optical system according to this embodiment is alsoconfigured to satisfy the conditional expressions (1) through (9) in theabove-described first embodiment. In this case, it is sufficient if atleast one of the cemented lens CL1 and the cemented lens CL2 isconfigured to satisfy each conditional expression.

EXAMPLES

Next, Examples 1 to 15 of the objective optical system according to anyone of the embodiments described above will be described with referenceto FIGS. 4 to 33. In the lens data described in each of the examples, ris the radius of curvature (mm), d is the axial distance (mm), Ndrepresents a refractive index at the d-line, and Vd represents the Abbenumber at the d-line.

Example 1

FIG. 4 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 1 of the presentinvention, FIG. 5 shows aberration charts, and lens data of theobjective optical system according Example 1 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.20 1.88300 40.76 2 0.462 0.27 3∞ 0.03 4 3.254 1.05 1.83400 37.16 5 −1.041  0.07 6 aperture stop 0.03 7∞ 0.60 1.52100 65.12 8 ∞ 0.10 9 2.124 0.85 1.75500 52.32 10 −0.804  0.301.95906 17.47 11 −2.068  0.38 12 ∞ 0.50 1.51633 64.14 13 ∞ 0.01 1.0000064.00 14 ∞ 0.50 1.00000 50.49 15 ∞ Various data Focal length 0.67 FNO.5.00 Angle of observation field 133.48 2ω Focal length of each groupFront group Rear group 2.25 1.93

Example 2

FIG. 6 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 2 of the presentinvention, FIGS. 7 and 8 show aberration charts, and lens data of theobjective optical system according Example 2 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.35 1.88300 40.76 2 1.158 0.85 3∞ 0.40 1.52100 65.12 4 ∞ 0.20 5 −3.355  1.70 1.58144 40.75 6 −2.430 0.30 7 5.659 0.80 1.51742 52.43 8 −1.284  0.30 1.92286 18.90 9 −2.002 0.05 10 aperture stop 0.03 11 ∞ D11 12 ∞ 0.30 1.77250 49.60 13 1.2160.55 1.72825 28.46 14 3.618 0.10 15 ∞ D15 16 4.765 1.15 1.81600 46.62 17−6.127  0.05 18 3.997 1.53 1.61800 63.33 19 −2.843  0.35 1.95906 17.4720 8.733 0.09 21 ∞ 0.10 22 ∞ 0.40 1.52300 58.59 23 ∞ 0.75 24 ∞ 0.751.51633 64.14 25 ∞ 0.01 1.51300 64.01 26 ∞ 0.65 1.50510 63.26 27 ∞ Zoomdata Normal View state Magnified view state Focal length 1.15 1.40 FNO.6.06 7.39 Angle of observation field 159.91 90.36 2ω Various data D110.31 1.71 D15 1.72 0.32 Focal length of each group First group Secondgroup Third group 1.94 −4.17 3.09

Example 3

FIG. 9 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 3 of the presentinvention, FIGS. 10 and 11 show aberration charts, and lens data of theobjective optical system according Example 3 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.35 1.88300 40.76 2 1.148 0.85 3∞ 0.40 1.52100 65.12 4 ∞ 0.20 5 −3.309  1.70 1.58144 40.75 6 −2.474 0.30 7 6.028 0.80 1.51742 52.43 8 −1.255  0.30 1.95906 17.47 9 −1.910 0.05 10 aperture stop 0.03 11 ∞ D11 12 ∞ 0.30 1.77250 49.60 13 1.0890.55 1.72825 28.46 14 3.739 0.10 15 ∞ D15 16 4.574 1.15 1.81600 46.62 17−6.626  0.05 18 3.758 1.53 1.61800 63.33 19 −2.858  0.35 1.95906 17.4720 6.853 0.09 21 ∞ 0.10 22 ∞ 0.40 1.52300 58.59 23 ∞ 0.76 24 ∞ 0.751.51633 64.14 25 ∞ 0.01 1.51300 64.01 26 ∞ 0.65 1.50510 63.26 27 ∞ Zoomdata Normal View state Magnified view state Focal length 1.15 1.40 FNO.6.21 7.49 Angle of observation field 160.04 90.53 2ω Various data D110.31 1.71 D15 1.72 0.32 Focal length of each group First group Secondgroup Third group 1.91 −4.20 3.12

Example 4

FIG. 12 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 4 of the presentinvention, FIGS. 13 and 14 show aberration charts, and lens data of theobjective optical system according Example 4 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.35 1.88300 40.76 2 1.139 0.85 3∞ 0.40 1.52100 65.12 4 ∞ 0.20 5 −3.688  1.52 1.58144 40.75 6 −2.511 0.30 7 19.344  0.80 1.58144 40.75 8 −1.041  0.30 1.95906 17.47 9 −1.693 0.05 10 aperture stop 0.03 11 ∞ D11 12 ∞ 0.30 1.77250 49.60 13 1.1240.55 1.72825 28.46 14 3.702 0.10 15 ∞ D15 16 4.569 1.15 1.81600 46.62 17−6.842  0.05 18 3.908 1.53 1.61800 63.33 19 −2.836  0.35 1.95906 17.4720 8.387 0.09 21 ∞ 0.10 22 ∞ 0.40 1.52300 58.59 23 ∞ 0.81 24 ∞ 0.751.51633 64.14 25 ∞ 0.01 1.51300 64.01 26 ∞ 0.65 1.50510 63.26 27 ∞ Zoomdata Normal View state Magnified view state Focal length 1.15 1.40 FNO.6.20 7.53 Angle of observation field 59.91 90.33 2ω Various data D110.31 1.71 D15 1.72 0.32 Focal length of each group First group Secondgroup Third group 1.92 −4.19 3.14

Example 5

FIG. 15 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 5 of the presentinvention, FIG. 16 shows aberration charts, and lens data of theobjective optical system according Example 5 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.27 1.88300 40.76 2 0.427 0.36 322.369 0.30 1.88300 40.76 4 1.152 0.68 1.69895 30.13 5 −0.908 0.05 6aperture stop 0.10 7 449.945 0.30 1.88300 40.76 8 1.223 0.82 1.4874970.23 9 −1.385 0.05 10 1.912 1.06 1.72916 54.68 11 −0.884 0.30 1.9590617.47 12 −1.771 0.05 13 ∞ 0.31 1.51400 85.67 14 ∞ 0.36 15 ∞ 0.30 1.5163364.14 16 ∞ 0.01 1.51300 64.01 17 ∞ 0.40 1.50510 63.26 18 ∞ Various dataFocal length 0.45 FNO. 3.00 Angle of observation field 2ω 159.51 Focallength of each group Front group Rear group −43.52 1.33

Example 6

FIG. 17 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 6 of the presentinvention, FIG. 18 shows aberration charts, and lens data of theobjective optical system according Example 6 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.25 1.88300 40.76 2  0.442 0.423 −5.140 0.70 1.69895 30.13 4 −0.900 0.05 5 ∞ 0.31 1.51400 85.67 6 ∞0.05 7 aperture stop 0.05 8 ∞ 0.25 1.88300 40.76 9  1.578 0.67 1.5163364.14 10 −1.264 0.03 11 ∞ 0.03 12 ∞ 0.03 13  1.572 0.78 1.72916 54.68 14−0.925 0.25 1.95906 17.47 15 −2.261 0.36 16 ∞ 0.30 1.51633 64.14 17 ∞0.52 1.50510 63.26 18 ∞ Various data Focal length 0.44 FNO. 2.99 Angleof observation field 2ω 162.55 Focal length of each group Front groupRear group −10.3449 1.2124

Example 7

FIG. 19 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 7 of the presentinvention, FIGS. 20 and 21 show aberration charts, and lens data of theobjective optical system according Example 7 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.30 1.88300 40.76 2  1.044 0.513 ∞ 0.40 1.52100 65.12 4 ∞ 0.19 5 −1.668 0.41 1.84666 23.78 6 −1.7560.10 7 ∞ 0.01 8 ∞ 0.02 9  2.070 0.35 1.84666 23.78 10  2.000 0.07 11 ∞1.18 12 ∞ 0.20 13 aperture stop 0.10 14  6.829 0.50 1.88300 40.76 15−2.715 0.31 1.71999 50.23 16 −5.630 0.31 17  2.888 0.89 1.72916 54.68 18−1.087 0.30 1.95906 17.47 19 −2.708 0.32 20 ∞ 0.40 1.52300 58.59 21 ∞0.02 22 ∞ 1.00 1.51633 64.14 23 ∞ 0.00 1.51300 64.01 24 ∞ 0.65 1.5051063.26 25 ∞ Zoom data Normal View state Magnified view state Focal length0.64 0.62 FNO. 3.00 3.00 Angle of observation 88.15 89.87 field 2ωVarious data D7 0.01 1.41 D11 1.18 0.01 Focal length of each group Firstgroup Second group Third group −1.43 54.09 1.73

Example 8

FIG. 22 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 8 of the presentinvention, FIG. 23 shows aberration charts, and lens data of theobjective optical system according Example 8 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.30 1.88300 40.76 2 0.427 0.52 32.314 0.30 1.88300 40.76 4 0.650 0.60 1.62004 36.26 5 −0.738  0.02 6aperture stop 0.01 7 ∞ 0.55 8 1.578 0.60 1.72916 54.68 9 −0.800  0.201.95906 17.47 10 −2.029  0.38 11 ∞ 0.50 1.51633 64.14 12 ∞ 0.00 1.5130064.01 13 ∞ 0.50 1.50510 63.26 14 ∞ Various data Focal length 0.48 FNO.2.98 Angle of observation field 2ω 132.22 Focal length of each groupFront group Rear group 2.55 1.65

Example 9

FIG. 24 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 9 of the presentinvention, FIG. 25 shows aberration charts, and lens data of theobjective optical system according Example 9 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.30 1.88300 40.76 2 0.427 0.54 32.638 0.30 1.88300 40.76 4 0.650 0.60 1.62004 36.26 5 −0.729 0.02 6aperture stop 0.01 7 ∞ 0.44 8 1.562 0.75 1.72916 54.68 9 −0.800 0.201.95906 17.47 10 −2.453 0.02 11 7.876 0.30 1.51633 64.14 12 −5.104 0.0613 ∞ 0.50 1.51633 64.14 14 ∞ 0.00 1.51300 64.01 15 ∞ 0.50 1.50510 63.2616 ∞ D16 Various data Focal length 0.44 FNO. 2.99 Angle of observationfield 2ω 174.40 Focal length of each group Front group Rear group 2.491.53

Example 10

FIG. 26 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 10 of the presentinvention, FIG. 27 shows aberration charts, and lens data of theobjective optical system according Example 10 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.20 1.88300 40.76 2  0.561 0.373 ∞ 0.31 1.51400 85.67 4 ∞ 0.03 5 ∞ 0.72 1.95906 17.47 6 −2.598 0.05 7aperture stop 0.10 8 ∞ 0.20 1.88300 40.76 9  1.020 0.70 1.69680 55.53 10−1.169 0.03 11  1.290 0.90 1.72916 54.68 12 −0.949 0.28 1.95906 17.47 13−7.770 0.21 14 ∞ 0.30 1.51633 64.14 15 ∞ 0.42 1.50510 63.26 16 ∞ Variousdata Focal length 0.44 FNO. 2.98 Angle of observation field 2ω 161.78Focal length of each group Front group Rear group −1.57 1.02

Example 11

FIG. 28 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 11 of the presentinvention, FIG. 29 shows aberration charts, and lens data of theobjective optical system according Example 11 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.20 1.88300 40.76 2  0.561 0.763 ∞ 0.40 1.95906 17.47 4 −2.691 0.05 5 aperture stop 0.17 6 ∞ 0.201.88300 40.76 7  1.020 0.70 1.69680 55.53 8 −1.135 0.03 9  1.269 0.901.72916 54.68 10 −0.949 0.28 1.95906 17.47 11 −22.869  0.21 12 ∞ 0.301.51633 64.14 13 ∞ 0.42 1.50510 63.26 14 ∞ Various data Focal length0.44 FNO. 2.98 Angle of observation field 2ω 161.76 Focal length of eachgroup Front group Rear group −1.47 1.02

Example 12

FIG. 30 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 12 of the presentinvention, FIG. 31 shows aberration charts, and lens data of theobjective optical system according Example 12 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.20 1.88300 40.76 2 0.558 0.80 3−21.491 0.40 1.95906 17.47 4 −2.925 0.05 5 aperture stop 0.25 6 4.3740.20 1.88300 40.76 7 1.019 0.70 1.69680 55.53 8 −1.346 0.03 9 1.331 0.901.72916 54.68 10 −0.980 0.28 1.95906 17.47 11 −36.237 0.21 12 ∞ 0.301.51633 64.14 13 ∞ 0.42 1.50510 63.26 14 ∞ Various data Focal length0.45 FNO. 2.98 Angle of observation field 2ω 159.66 Focal length of eachgroup Front group Rear group −1.21 1.04

Example 13

FIG. 32 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 13 of the presentinvention, FIGS. 33 and 34 show aberration charts, and lens data of theobjective optical system according Example 13 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.35 1.88300 40.76 2 1.108 1.18 3−2.043 1.57 1.72916 54.68 4 −2.391 0.05 5 5.855 1.03 1.77250 49.60 6−2.460 0.34 1.92286 18.90 7 −4.244 0.20 8 aperture stop 0.03 9 ∞ 0.301.77250 49.60 10 1.358 0.50 1.59270 35.31 11 9.321 1.90 12 4.364 1.401.48749 70.23 13 −3.267 0.05 14 5.198 1.70 1.48749 70.23 15 −2.130 0.241.95906 17.47 16 −5.691 0.30 17 ∞ 0.03 18 ∞ 0.40 1.52300 28.59 19 ∞ 0.7220 ∞ 0.75 1.51633 64.14 21 ∞ 0.01 1.51300 64.01 22 ∞ 0.65 1.50510 63.2623 ∞ Zoom data Normal View state Magnified view state Focal length 1.111.40 FNO. 7.62 7.37 Angle of observation 159.99 90.11 field 2ω Variousdata D7 0.20 1.80 D11 1.90 0.30 Focal length of each group First groupSecond group Third group 2.06 −5.03 3.37

Example 14

FIG. 35 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 14 of the presentinvention, FIGS. 36 and 37 show aberration charts, and lens data of theobjective optical system according Example 14 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.35 1.88300 40.76 2 1.108 1.16 3−2.043 1.60 1.72916 54.68 4 −2.419 0.05 5 5.700 1.03 1.77250 49.60 6−2.629 0.34 1.95906 17.47 7 −4.273 0.20 8 aperture stop 0.03 9 ∞ 0.301.77250 49.60 10 1.352 0.50 1.59270 35.31 11 9.392 1.90 12 4.363 1.401.48749 70.23 13 −3.271 0.05 14 5.197 1.70 1.48749 70.23 15 −2.130 0.241.95906 17.47 16 −5.686 0.30 17 ∞ 0.03 18 ∞ 0.40 1.52300 58.59 19 ∞ 0.7220 ∞ 0.75 1.51633 64.14 21 ∞ 0.01 1.51300 64.01 22 ∞ 0.65 1.50510 63.2623 ∞ Zoom data Normal View state Magnified view state Focal length 1.111.40 FNO. 7.62 7.37 Angle of observation 159.98 90.12 field 2ω Variousdata D7 0.20 1.80 D11 1.90 0.30 Focal length of each group First groupSecond group Third group 2.06 −5.03 3.37

Example 15

FIG. 38 shows a cross-sectional view of the overall configuration of anobjective optical system according to Example 15 of the presentinvention, FIGS. 39 and 40 show aberration charts, and lens data of theobjective optical system according Example 15 is shown below.

Lens data Surface Number r d Nd Vd 1 ∞ 0.35 1.88300 40.76 2 1.108 1.17 3−2.100 1.64 1.72916 54.68 4 −2.340 0.05 5 5.895 1.03 1.77250 49.60 6−2.751 0.34 1.95906 17.47 7 −4.576 0.20 8 aperture stop 0.03 9 −17.9510.30 1.77250 49.60 10 1.424 0.50 1.59270 35.31 11 22.538 1.90 12 4.6481.40 1.48749 70.23 13 −3.193 0.05 14 5.299 1.70 1.48749 70.23 15 −2.1300.24 1.95906 17.47 16 −5.590 1.38 17 ∞ 0.75 1.51633 64.14 18 ∞ 0.011.51300 64.01 19 ∞ 0.65 1.50510 63.26 20 ∞ Zoom data Normal View stateMagnified view state Focal length 1.10 1.40 FNO. 7.55 7.32 Angle ofobservation 159.99 90.12 field 2ω Various data D7 0.20 1.80 D11 1.900.30 Focal length of each group First group Second group Third group2.06 −5.05 3.40

The values according to the aforementioned expressions (1) through (10)are shown in Table 1 and Table 2.

TABLE 1 EXAMPLE EXAMPLE EXAMPLE EXAMPLE EXAMPLE EXAMPLE EXAMPLE EXAMPLEEXAMPLE 1 2 3 4 5 6 7 8 9 νA − ndA 15.51 15.51 15.51 15.51 15.51 15.5115.51 15.51 15.51 rdyA1/ih −1.24 −2.47 −2.49 −2.47 −1.97 −2.06 −2.42−1.78 −1.78 (rdyA1 + rdyA2)/ −2.27 −0.51 −0.41 −0.49 −2.99 −2.39 −2.34−2.30 −1.97 (rdyA1 − rdyA2) rdy12/rdyA1 −0.57 −0.41 −0.40 −0.40 −0.48−0.48 −0.96 −0.53 −0.53 FL2G*2Δ/fl{circumflex over ( )}2 — 4.48 4.524.48 — — 155.21 — — DB/DA 2.83 4.37 4.37 4.37 3.53 3.12 2.96 3.00 3.75DA/ih 0.46 0.30 0.30 0.30 0.67 0.56 0.67 0.45 0.45 PW1/PW4 2.97 1.681.59 1.69 4.56 3.59 1.76 3.09 2.72 (rdy11 + rdy12)/ 1.00 1.00 1.00 1.001.00 1.00 1.00 1.00 1.00 (rdy11 − rdy12) (rdyB1 + rdyB2)/ 0.45 0.17 0.140.16 0.37 0.26 0.45 0.33 0.32 (rdyB1 − rdyB2)

TABLE 2 EXAM- EXAM- EXAM- EXAM- EXAM- EXAM- PLE 10 PLE 11 PLE 12 PLE 13PLE 14 PLE 15 νA − ndA 15.51 15.51 15.51 15.51 15.51 15.51 rdyA1/ih−2.11 −2.11 −2.18 −1.85 −1.85 −1.85 (rdyA1 + −1.28 −1.09 −1.06 −2.20−2.20 −2.23 rdyA2)/ (rdyA1 − rdyA2) rdy12/rdyA1 −1.69 −1.69 −1.75 −1.92−1.92 −1.92 FL2G*2Δ/fl{circumflex over ( )}2 — — — 5.91 5.91 6.03 DB/DA3.27 3.27 3.27 7.00 7.00 7.00 DA/ih 0.61 0.61 0.61 0.21 0.21 0.21PW1/PW4 1.81 1.64 1.67 2.93 2.93 2.96 (rdy11 + 1.00 1.00 1.00 1.00 1.001.00 rdy12)/ (rdy11 − rdy12) (rdyB1 + 0.15 0.14 0.15 0.42 0.42 0.43rdyB2)/ (rdyB1 − rdyB2)

The inventors have arrived at the following aspects of the invention.

One aspect of the present invention is an endoscope objective opticalsystem including at least a first cemented lens which has a positivelens and a negative lens, in which the cemented lens satisfies thefollowing conditional expressions,

15.0<νA−ndA<15.75  (1)

−0.2>rdyA1/ih>−20  (2)

wherein νA is an Abbe number of the negative lens, ndA is a refractiveindex of the negative lens at the d-line, rdyA1 is a curvature radius ofa joining surface of the negative lens, and ih is an image height.

According to this aspect, axial chromatic aberration and chromaticaberration of magnification can be favorably corrected using at leastthe first cemented lens that has the positive lens and the negative lensand that satisfies the conditional expression (1) and the conditionalexpression (2) simultaneously.

In the above-described aspect, it is preferable that the negative lensof the first cemented lens satisfies the following conditionalexpression,

−0.2>(rdyA1+rdyA2)/(rdyA1−rdyA2)>−10  (3)

wherein rdyA1 is a curvature radius of the joining surface of thenegative lens of the first cemented lens, and rdyA2 is a curvatureradius of an air-contacting surface of the negative lens of the firstcemented lens.

By doing so, axial and non-axial chromatic aberrations can be correctedwhile still achieving the necessary negative refracting power.

It is preferable that the optical system of the above-described aspecthas a front lens group, an aperture stop, and a rear lens group arrangedin this order from an object, that the rear lens group has a positiverefractive index, and that the first cemented lens is disposed in atleast one of the front lens group and the rear lens group.

By doing so, the number of lenses of each group can be reduced, theoverall length of the endoscope objective optical system can beshortened, and the cost can be reduced. In addition, a long back focuscan be secured while still restraining the size in the lens radialdirection. Furthermore, axial chromatic aberration and chromaticaberration of magnification can be favorably corrected by providing thecemented lens in at least one of the front lens group and the rear lensgroup.

In the above-described aspect, it is preferable that the front lensgroup includes a negative lens and a positive lens arranged in thisorder from the object and that the rear lens group includes the firstcemented lens.

By doing so, the number of lenses of each group can be reduced, theoverall length of the endoscope objective optical system can beshortened, and the cost can be reduced. In addition, a long back focuscan be secured while still restraining the size in the lens radialdirection. In addition, axial chromatic aberration and chromaticaberration of magnification can be favorably corrected by providing thecemented lens in the rear lens group.

In the above-described aspect, it is preferable that the rear lens grouphas a plurality of the first cemented lenses.

By doing so, the number of lenses of each group can be reduced, theoverall length of the endoscope objective optical system can beshortened, and the cost can be reduced. In addition, a long back focuscan be secured while still restraining the size in the lens radialdirection. Furthermore, axial chromatic aberration and chromaticaberration of magnification can be favorably corrected by providing thecemented lens in the rear lens group.

In the above-described aspect, it is preferable that the front lensgroup has a negative lens and a cemented lens formed by joining at leastone positive lens and at least one negative lens arranged in this orderfrom the object, that the rear lens group has a plurality of thecemented lenses, and that the first cemented lens is provided in atleast one of the front lens group and the rear lens group.

By doing so, the number of lenses of each group can be reduced, theoverall length of the endoscope objective optical system can beshortened, and the cost can be reduced. In addition, a long back focuscan be secured while still restraining the size in the lens radialdirection. Furthermore, axial chromatic aberration and chromaticaberration of magnification can be favorably corrected by providing thecemented lens in the rear lens group.

In the above-described aspect, it is preferable that the front lensgroup has a negative lens and a cemented lens formed by joining at leastone positive lens and at least one negative lens arranged in this orderfrom the object, that the rear lens group has the cemented lens and apositive lens, and that at least one of the front lens group and therear lens group has the first cemented lens.

By doing so, the number of lenses of each group can be reduced, theoverall length of the endoscope objective optical system can beshortened, and the cost can be reduced. In addition, a long back focuscan be secured while still restraining the size in the lens radialdirection. Furthermore, axial chromatic aberration and chromaticaberration of magnification can be favorably corrected by providing thecemented lens in the rear lens group.

Furthermore, by providing the positive lens closest to the image side,the exit ray angle can be made gentle, thereby making it possible tocorrect shading favorably.

In the above-described aspect, it is preferable that the optical systemincludes a positive first lens group, a movable negative second lensgroup, and a positive third lens group arranged in this order from anobject, that it is possible to switch between a normal view and amagnified view by moving the second lens group, and that the third lensgroup includes the at least one first cemented lens.

By doing so, the number of lenses of each group can be reduced, theoverall length of the endoscope objective optical system can beshortened, and the cost can be reduced, while still ensuring a wideangle of observation field and making it possible to achieve a goodfocus between the range from the normal view to the magnified view.

In the above-described aspect, it is preferable that the optical systemincludes a negative first lens group, a second lens group movable at thetime of focusing, and a positive third lens group arranged in this orderfrom an object, that it is possible to switch between a normal view anda magnified view by moving the second lens group, and that the thirdlens group has the at least one first cemented lens.

By doing so, a variation in aberration at the time of focusing can bereduced while still ensuring a long back focus, making it possible toproduce an objective optical system that is tolerant to manufacturingerrors. In addition, axial chromatic aberration and chromatic aberrationof magnification can be favorably corrected as a result of the at leastone cemented lens being disposed in the third lens group. It ispreferable that the second lens group has positive refracting power ornegative refracting power.

In the above-described aspect, it is preferable that the negative firstlens is disposed closest to the object and satisfies the followingconditional expression,

−3.0≤rdy12/rdyA1<−0.2  (4)

wherein rdy12 is an image-side curvature radius of the negative firstlens, and rdyA1 is a curvature radius of a joining surface of thenegative lens of the first cemented lens.

The conditional expression (4) is a conditional expression for theimage-side curvature radius of the negative first lens and the curvatureradius of the cemented lens. By satisfying the conditional expression(4), it is possible to favorably maintain the balance between theimage-side curvature radius of the negative first lens and the curvatureradius of the cemented lens, making it possible to favorably correctcomatic aberration, field curvature, axial chromatic aberration, andchromatic aberration of magnification.

In the above-described aspect, it is preferable that the first cementedlens satisfies the following conditional expressions,

1.0<DB/DA<10  (5)

0.1<DA/ih<2.0  (6)

wherein DA is a thickness at the middle of the negative lens in thefirst cemented lens, DB is a thickness at the middle of the positivelens in the first cemented lens, and ih is an image height.

The conditional expression (5) and conditional expression (6) areconditional expressions regarding the thickness at the middle of thecemented lens. By satisfying the conditional expression (5) andconditional expression (6), it is possible to achieve an endoscopeobjective optical system with an appropriate overall length in which thelenses do not easily exhibit manufacturing defects, such as fracture andcracks.

In the above-described aspect, it is preferable that the followingconditional expression is satisfied,

0.5<PW1/PW4<10  (7)

Wherein, PW1 is a refracting power of the negative first lens, and PW4is a refracting power of the negative lens in the first cemented lens.

By satisfying the conditional expression (7), it is possible tofavorably maintain the balance between the refracting power of thenegative first lens and the refracting power of the negative lens of thefirst cemented lens, making it possible to favorably correct comaticaberration, field curvature, axial chromatic aberration, and chromaticaberration of magnification.

In the above-described aspect, it is preferable that the followingconditional expression is satisfied,

0.5<(rdy11+rdy12)/(rdy11−rdy12)<1.7  (8)

wherein rdy11 is an object-side curvature radius of the negative firstlens.

The necessary negative refracting power can be obtained by satisfyingthe conditional expression (8).

In the above-described aspect, it is preferable that the followingconditional expression is satisfied,

0.05<(rdyB1+rdyB2)/(rdyB1−rdyB2)<2.0  (9)

wherein rdyB1 is a curvature radius of an air-contacting surface of thepositive lens in the first cemented lens, and rdyB2 is a curvatureradius of a joining surface of the positive lens in the first cementedlens.

By satisfying the conditional expression (9), an appropriate curvatureradius can be derived, and the thickness of the lens at the peripherythereof can be secured while still ensuring the necessary positiverefracting power.

In the above-described aspect, it is preferable that the followingconditional expression is satisfied,

1<FL2G×Δ2G/FL²<200  (10)

wherein Δ2G is an absolute value of displacement of the second lensgroup from a normal view state to a close-up magnified view state, FL isa focal length of the entire objective optical system in the normal viewstate, and FL2G is a focal length of the second lens group.

By satisfying the conditional expression (10), an appropriatedisplacement can be performed and a focus stroke in accordance withtechnician's feeling can be realized.

ADVANTAGEOUS EFFECTS OF INVENTION

The aforementioned aspects affords an advantage in that an image havinghigh precision and a wide angle of observation field can be obtained byfavorably correcting various aberrations while ensuring lowinvasiveness.

REFERENCE SIGNS LIST

-   G1 first group-   G2 second group-   L1 first lens-   L2 second lens-   L3 third lens-   L4 fourth lens-   L5 fifth lens-   L6 sixth lens-   L7 seventh lens-   CL1 cemented lens

1. An endoscope objective optical system comprising a front lens group,an aperture stop, and a rear lens group arranged in this order from anobject, wherein the front lens group has a negative refractive power ora positive refractive power, and the rear lens group has a positiverefractive power, wherein the front lens group includes a negative lensand a positive lens arranged in this order from the object, wherein therear lens group includes a cemented lens which has a positive lens and anegative lens, the cemented lens satisfies the following conditionalexpressions,15.5<νA−ndA<15.6  (1)″−1.2>rdyA1/ih>−20  (2−1) wherein νA is an Abbe number of the negativelens of the cemented lens, ndA is a refractive index of the negativelens of the cemented lens at the d-line, rdyA1 is a curvature radius ofa joining surface of the negative lens of the cemented lens, and ih isan image height.
 2. The endoscope objective optical system according toclaim 1, wherein the negative lens of the cemented lens satisfies thefollowing conditional expression,−0.2>(rdyA1+rdyA2)/(rdyA1−rdyA2)>−10  (3) wherein rdyA1 is a curvatureradius of the joining surface of the negative lens of the cemented lens,and rdyA2 is a curvature radius of an air-contacting surface of thenegative lens of the cemented lens.
 3. The endoscope objective opticalsystem according to claim 1, wherein a negative first lens is disposedclosest to the object and satisfies the following conditionalexpression,−3.0≤rdy12/rdyA1<−0.2  (4) wherein rdy12 is an image-side curvatureradius of the negative first lens, and rdyA1 is a curvature radius of ajoining surface of the negative lens of the cemented lens.
 4. Theendoscope objective optical system according to claim 1, wherein thecemented lens satisfies the following conditional expressions,1.0<DB/DA<10  (5)0.1<DA/ih<2.0  (6) wherein DA is a thickness at the middle of thenegative lens in the cemented lens, DB is a thickness at the middle ofthe positive lens in the cemented lens, and ih is an image height. 5.The endoscope objective optical system according to claim 3, wherein theoptical system satisfies the following conditional expression,0.5<PW1/PW4<10  (7) wherein PW1 is a refracting power of the negativefirst lens, and PW4 is a refracting power of the negative lens in thecemented lens.
 6. The endoscope objective optical system according toclaim 3, wherein the optical system satisfies the following conditionalexpression,0.5<(rdy11+rdy12)/(rdy11−rdy12)<1.7  (8) wherein rdy11 is an object-sidecurvature radius of the negative first lens, and rdy12 is an image-sidecurvature radius of the negative first lens.
 7. The endoscope objectiveoptical system according to claim 3, wherein the optical systemsatisfies the following conditional expression,0.05<(rdyB1+rdyB2)/(rdyB1−rdyB2)<2.0  (9) wherein rdyB1 is a curvatureradius of an air-contacting surface of the positive lens in the cementedlens, and rdyB2 is a curvature radius of a joining surface of thepositive lens in the cemented lens.